1. Field of the Invention
This invention relates to diagnosing internal combustion engines electronically.
2. Description of the Prior Art
The diagnosing of the health of an internal combustion engine, and particularly its dynamic operating characteristics, requires knowledge of engine speed. In the prior art, there are a wide variety of known techniques for measuring speed. Perhaps the best known is the electronic tachometer which utilizes electrical signals from the distributor of a spark-ignition internal combustion engine. Such tachometers are subject to erratic operation due to large electrical noise components in the signals being monitored, and rely on the proper operation of the distributor for accuracy, particularly on a sub-cyclic basis. And, the electrical health of the engine may be the precise problem which is desired to be checked, so the distributor may create faulty speed indications which might confuse the diagnostics involved. In engines which do not utilize a spark ignition, such as diesel engines, it is even more difficult to provide adequate speed indications. For average speed measurements, when only a coarse indication of speed is required (as in the case of operation of the vehicle, or in the case of checking for general low idle or high idle speeds) speed measurements on a cyclic basis (based on one signal per revolution or per cycle of the engine) are adequate. But in certain diagnostic procedures, these are inadequate.
As an example, consider the known technique of measuring acceleration of an engine to provide indications related to torque, when then may be related through speed to horsepower, to provide a general indication of engine health. In one technique known to the art, the engine is allowed to undergo a burst acceleration from a low speed to a high speed, the speed of the engine is monitored, and the elapsed time between the sensing of first and second speeds is measured. The problem in this technique is that the speed measurement is made over an entire engine cycle, so the precise time at which the engine crosses the lower threshold speed and the upper threshold speed, thereby to accurately measure the time interval required for the engine to accelerate from one speed to another, cannot be known. In this technique, interpolation based on the difference between the average speeds sensed over succeeding cycles and the desired threshold speeds is utilized to correct the time increment otherwise measured between cycles which follow the sensing of threshold speeds.